1. Field of the Invention
The present disclosure relates to methods of making ferritic stainless steels having at least one oxidation resistant surface. The present disclosure also relates to ferritic stainless steels including at least one oxidation resistant surface, and further relates to articles of manufacture formed of or including such ferritic stainless steels.
2. Description of the Invention Background
Fuel cells are energy conversion devices that generate electricity and heat by electrochemically combining a gaseous fuel and an oxidizing gas via an ion-conducting electrolyte. A primary feature of fuel cells is the ability to convert chemical energy directly into electrical energy in the absence of combustion, which provides significantly higher conversion efficiencies than reciprocating engines, gas turbines and other conventional thermomechanical methods of producing energy. For the same power output, fuel cells produce substantially less carbon dioxide emissions than technologies based on fossil fuels. Fuel cells also produce negligible amounts of SOx and NOx, which are the main constituents of acid rain and photochemical smog.
Several types of fuel cells are currently being developed. A primary difference between these fuel cell types is the material utilized as the electrolyte, which effects operating temperature. NASA developed alkaline fuel cells including a liquid electrolyte in the 1960's to power Apollo and other spacecraft, and NASA currently uses greatly improved versions on the Space Shuttle. Solid oxide fuel cells (SOFCs), in contrast, are constructed entirely of solid-state materials, using a fast oxygen ion-conducting ceramic (typically yttria-stabilized zirconia or “YSZ”) as the electrolyte, and operate in a temperature range of about 500° C. (932° F.) to 1000° C. (1832° F.) to facilitate solid-state transport. Advantages of SOFCs relative to other fuel cell types include high energy efficiency and few problems with electrolyte management (liquid electrolytes are typically corrosive and may be difficult to handle). SOFCs also produce high-grade waste heat, which can be used in combined heat and power devices, and internal reforming of hydrocarbon fuels (to produce hydrogen and methane) is possible.
An organization promoting development of SOFCs in the United States is the Solid State Energy Conversion Alliance (SECA). SECA consists of an Industry Group, which is focused on building integrated SOFCs using technologies developed by the SECA member companies, and a Core Technology Group, which carries out fundamental research driven by the needs of the Industry Group as a whole. The SECA program, which was organized and is overseen by the United States Department of Energy's Office of Fossil Energy, has set certain cost and performance goals for SOFCs under development.
A single SOFC “cell” or subunit includes an anode and a cathode, which are separated by the electrolyte. Because current generation SOFCs operate at temperatures up to about 1000° C. (about 1832° F.), the electrodes are generally constructed from ceramic materials to avoid environmental degradation. Both the anode and cathode layers are intentionally permeable to gases via the establishment of a network of interconnected porosity and are good electrical conductors (e.g., they exhibit essentially no ionic conductivity). In current generation SOFCs, the anode is typically formed from an electrically conductive nickel/YSZ composite (a ceramic/metal composite or “cermet”). The nickel provides a continuous electrically conductive path, while the YSZ serves to reduce the coefficient of thermal expansion of the overall composite and prevents the porosity from sintering shut. The nickel particles are protected from oxidation by the hydrogen-rich gas present at the anode, which is reducing to pure nickel. The cathode may be based on, for example, lanthanum manganate (LaMnO3), typically doped with strontium (replacing some of the lanthanum to yield La1-xSrxMnO3) to improve its electrical conductivity. The electrolyte is typically a thin (relative to the anode and cathode) layer of fully dense YSZ.
During operation of the SOFC cell, an oxidant (such as O2 or air) is fed into the fuel cell on the cathode side, where it supplies oxygen ions to the electrolyte by accepting electrons from an external circuit by the following half-cell reaction:½O2(g)+2e−=O−2 The oxygen atoms pass through the YSZ electrolyte via solid state diffusion to the electrolyte/anode interface. The SOFC can employ hydrogen (H2) and/or carbon monoxide (CO) as a basic fuel. Operationally, pure hydrogen can be used as supplied, while a hydrocarbon fuel such as methane, kerosene, or gasoline must be partially combusted, or reformed, to hydrogen and carbon monoxide. This is typically accomplished internally within the fuel cell, aided by the high operating temperature and by steam injection. The fuel gas mixture penetrates the anode to the anode/electrolyte interface, where it reacts with the oxygen ions from the ion-conducting electrolyte in the following two half-cell reactions:H2(g)+O−2=2e−+H2O(g) CO(g)+O−2=2e−+CO2(g) This releases electrons, which re-enter the external circuit. The flow of electrical charge due to oxygen ion transport through the electrolyte from cathode to anode is balanced exactly by the flow of electrical charge due to electron conduction in the external circuit. The driving force is the need to maintain overall electrical charge balance. The flow of electrons in the external circuit provides useful power at a potential of approximately one volt.
To generate a reasonable voltage, fuel cells are not operated as single units but, instead, as “stacks” composed of a series arrangement of several individual cells with an “interconnect” joining and conducting current between the anode and cathode of immediately adjacent cells. A common stack design is the flat-plate or “planar” SOFC (PSOFC), which is shown in a schematic form in FIG. 1. In the PSOFC 10 of FIG. 1, a single energy conversion cell 12 includes cathode 20 and anode 30 separated by electrolyte 40. Interconnect 50 separates anode 30 from cathode 60 of an immediately adjacent energy conversion cell 14 (not fully shown) within the stack. PSOFC 10 includes a repeating arrangement of cells identical to cell 12, with an interconnect disposed between each adjacent unit.
The design of a SOFC interconnect is critical because the interconnect serves several functions, including separating and containing the reactant gases and providing a low resistance path for current so as to electrically connect the cells in series. In general, the interconnect material must withstand the harsh high-temperature environmental conditions within the cells; must be suitably electrically conductive (including any oxides or other surface film or scale corrosion that forms on the material); and must have a coefficient of thermal expansion (CTE) that is sufficiently similar to the CTE of the ceramic electrodes within the cells to ensure the requisite structural integrity and gas-tightness of the fuel cell stack.
Initial PSOFC designs used a doped lanthanum chromate (LaCrO3) ceramic material as the interconnect material. LaCrO3 ceramic does not degrade at the high temperatures at which SOFCs operate and has a coefficient of thermal expansion that substantially matches the other ceramic components of the fuel cell. LaCrO3 ceramic, however, is brittle, difficult to fabricate, and extremely expensive. To address these deficiencies, metallic interconnects have been proposed for use in PSOFCs. These include interconnects formed from nickel-base alloys, such as AL 600™ alloy, and certain austenitic stainless steels, such as the 300 series family, the prototype of which is Type 304 alloy. Ferritic stainless steels, such as ALFA-II™ alloy, E-BRITE® alloy and AL 453™ alloy also have been proposed for use in PSOFC interconnects. Table 1 provides nominal compositions for the foregoing nickel-base and stainless steel alloys, all of which are available from Allegheny Ludlum Corporation, Pittsburgh, Pa.
TABLE 1Composition (weight percent)AlloyNiCrFeAlSiMnOtherAL 453 ™ 0.3 max.22bal.0.60.30.30.06alloyCe + Lamax.E-BRITE ®0.15 max.26bal.0.10.20.051 MoalloyALFA-II ™ 0.3 max.13bal.30.30.40.4 TialloyAL 600 ™bal.15.58—0.20.25—alloyType 304818bal.————alloy
Ferritic stainless steels have certain properties that make them attractive for PSOFC interconnect applications, including low cost, good fabricability, and CTE compatible with ceramic. Metallic interconnects, however, generally tend to form a surface oxide layer with low electrical conductivity at the high temperatures typical of PSOFC operation. This layer grows thicker with time and increases the resistivity of the interconnects and of the PSOFC stack as a whole. Certain alloys, upon exposure to oxygen at high temperatures, form surface oxides that thicken at an extremely slow rate (for example, the Al2O3 scale of ALFA-II® alloy) or are highly electrically conductive (for example, the NiO scale of pure or dispersion-strengthened nickel). However, the underlying mechanism that controls these two seemingly disparate factors (resistivity and rate of oxide formation) is essentially the same (the electronic defect structure of the oxide), resulting in very few oxides that are both slow growing and electrically conductive.
Ferritic stainless steels have attracted interest as interconnect material in part because in their conventional form they develop a scale consisting primarily of chromium oxide (Cr2O3), which is both relatively slow-growing and electrically conductive at high temperatures. Heat-resistant alloys that rely on oxidation of chromium may offer a compromise between relatively slow oxide scale growth and appreciable electrical conductivity at high temperature. Nevertheless, the rate of oxidative degradation of commercially available chromia-forming ferritic stainless steels would result in degradation of fuel stack performance over time. Alternative, non-metallic materials from which interconnects may be constructed, particularly perovskite ceramics (such as LaCrO3), do not exhibit similar levels of degradation, but increase the cost of the PSOFC stack to uneconomical levels.
Accordingly, there is a need for novel fuel cell interconnect material that is both economical and exhibits a suitable level of oxidation resistance when subjected to PSOFC operating conditions over time. More generally, there is a need for a ferritic stainless steel having improved oxidation resistance when exposed at high temperatures to an oxidizing environment.